A wind indicator, called an anemometer, is an instrument used to measure the wind’s speed, direction, or both.
A mechanical anemometer’s speed sensor consists of three small cups that spin around a vertical axis when the wind blows. The speed sensor measures how many times the cups rotate during a specific period of time, and converts this calculation into wind speed.
The vane sits upright, like a rudder, and moves horizontally when the wind changes direction.
Information from the vane and speed sensor is sent to a gauge/weather station for viewing and/or recording.
Unlike its mechanical predecessor, an ultrasonic/electronic anemometer has no moving parts to wear out, freeze up or fail. Instead, an electronic anemometer uses ultrasonic or laser technology to sample the atmosphere many, many times per second, and translates this input into useable wind characteristic data that’s displayed on a screen (often in conjunction with other meteorological information).
The most obvious way to use an anemometer is in a weather station context, but knowing the wind’s speed and direction is tremendously helpful when you’re out on a sailboat.